Recently, the manufacturing processes for electronic components are more and more precise and their sizes also have become smaller and smaller. Hence, the devices used to protect electronic components from the damage resulting from electric effects, such as static electricity, overvoltage, electric arc and so on, also become more and more important. For instance, the thyristor overvoltage protection devices are used in the modern communication systems extensively. These devices are used to protect the communication system from the damage resulting from lighting strikes on transmission lines, short circuits of neighboring power lines or other unexpected events. These devices can prevent any damage resulting from the overvoltage effects.
The thyristor overvoltage protection device is a semiconductor device designed to lead the overvoltage surge away from the transmission line before it reaches the communication system. Hence, it can be used to protect the communication system. When the system operates regularly, this protection device is kept in a high-resistance status, i.e. in an off status. At this moment, only the leakage current, which is lower than a microampere, can pass through this device. Hence, it won't affect the operation of the whole system. When an overvoltage surge occurs on the transmission lines, this device will switch to a low-resistance status, i.e. an on status. Thereby, this device can lead the overvoltage surge away from the communication system. After the overvoltage surge passes away, the overvoltage protection device will switch back to the off status and the communication system will return to regular operations.
The characteristics of the current and voltage of this overvoltage protection device are shown in FIG. 1, which is a curve diagram of the voltage (V) versus the current (I) of the conventional overvoltage protection device.
In general, the thyristor overvoltage protection device has two metal electrodes and a four-layer interleaving semiconductor structure, for example, which is interleaved by NPNP-type or PNPN-type layers. The top layer of this protection device is an emitter region, i.e. the cathode region; the second layer is a base region; the third layer is a substrate region; and the fourth layer is an anode region. Therein, the two metal electrodes are disposed on the surfaces of the emitter region and the anode region, respectively.
The junction between the base region and the substrate region is the central junction of this protection device. Under regular operation, the central junction will be reverse biased. When the reverse bias increases, the central junction will breakdown. As shown in the figure, when the breakdown current reaches 1 mA, the voltage across the protection device is defined as the breakdown voltage (Vz). If the voltage increases constantly at this moment, the breakdown current will increase rapidly and make the protection device switch to the on status. The voltage and current for making the protection switch to the on status are defined as the breakover voltage (VBO) and the breakover current (IBO). When the overvoltage surge occurs and reaches the breakover voltage (VBO), the overvoltage protection device will turn on to lead the induced current through the protection device and keep the voltage across the protection device in a relatively low value. When the overvoltage surge passes away, the current passing through the protection device will decrease constantly. When the current is lower than the holding current (IH) of the protection device, the overvoltage protection device will switch back to the off status (as shown in the figure) to make the voltage across the protection device return to normal and make the communication system operate regularly.
Reference is made to FIG. 2, which is a schematic diagram of a overvoltage protection device disclosed in U.S. Pat. No. 4,967,256. The thyristor overvoltage protection device has a four-layer semiconductor structure (PNPN), including emitter regions 22 (n++), shorting dots 23 disposed between the emitter regions 22, a substrate 20, a single buried region 25 (n) disposed inside the substrate 20, a base region 21 (p+), an anode region 24, a first metal electrode region 26 connected with the upper components, a second metal electrode region 27 connected with the lower components and a guard ring 28 (n++) surrounding the central junction. The guard ring 28 is used to make the potential difference of the component surface evenly distributed to improve the stability of the whole device. Further, the guard ring 28 won't result in the breakdown of this semiconductor device.
As shown in FIG. 2, the buried region 25 and the substrate 20 both are N-type semiconductors, but the buried region 25 has a higher impurity concentration. The breakdown voltage of the junction between the buried region 25 and the substrate 20 is lower than that between the base region 21 and the substrate 20. Hence, when the voltage across the protection device increases, the breakdown effect will first occur at the junction between the base region 21 and the buried region 25 and make the breakdown current pass through the buried region 25 first. This effect can improve the precision for controlling the breakdown voltage during manufacturing process of the device. Further, it can make this device have a breakover current even lower than that of the traditional device without the buried region.
However, in application, this overvoltage protection device still has drawbacks. Since it employs a single buried region with relative small size, its conductivity will be limited during the on status. Hence, it will cause a bottleneck effect, which will lower the current carrying capacity of the device. In order to prevent the bottleneck effect, U.S. Pat. No. 5,001,537 and No. 5,516,705 disclose two kinds of overvoltage protection devices employing multiple buried regions. However, since the operation principle of the devices is still unchanged, i.e., as described above, controlling the breakdown voltage and breakover current via employing the effect that the junction between the base region and the buried region will breakdown first, which is different to the present invention.
Accordingly, the present invention disposes a voltage-limiting region parallel to the central junction of the overvoltage protection device during the manufacturing process of the semiconductor device for defining the breakdown voltage and the breakover current of the device. Thereby, the present invention can provide a precise overvoltage protection device. Since it doesn't have the buried region, it can have the higher current carrying capacity.